The present invention relates to the field of power over Ethernet and more particularly to a power over Ethernet controller exhibiting a plurality of pre-defined current limits each available for each port of the power over Ethernet controller.
The growth of local and wide area networks based on Ethernet technology has been an important driver for cabling offices and homes with structured cabling systems having multiple twisted wire pairs. The ubiquitous local area network, and the equipment which operates thereon, has led to a situation where there is often a need to attach a network operated device for which power is to be advantageously supplied by the network over the network wiring. Supplying power over the network wiring has many advantages including, but not limited to; reduced cost of installation; centralized power and power back-up; and centralized security and management.
Several patents addressed to this issue exist including: U.S. Pat. No. 6,473,608 issued to Lehr et al., whose contents are incorporated herein by reference and U.S. Pat. No. 6,643,566 issued to Lehr et al., whose contents are incorporated herein by reference. Furthermore a standard addressed to the issue of powering remote devices over an Ethernet based network has been published as IEEE 802.3af-2003, whose contents are incorporated herein by reference, and is referred to hereinafter as the “af” standard. A device receiving power over the network wiring is referred to as a powered device (PD) and the powering equipment delivering power into the network wiring for use by the PD is referred to as a power sourcing equipment (PSE).
Commercially available PSEs are supplied supporting a single port, or supporting a plurality of ports. For clarity, each port of a power over Ethernet (PoE) controller serving a plurality of ports is referred to as a PSE and exhibits electrical characteristics as described in the “af” standard. In particular a current limit, denoted hereinafter as Ilimit, is defined that is not to be exceeded, even in the event of a short circuit, and is preferably also used to limit inrush current of the PD, defined as Iinrush.
An additional overload limit lower than Ilimit is defined for each port and denoted hereinafter as Icut. Current through the port may exceed Icut, provided that Ilimit is not exceeded, for a specified amount of time, denoted hereinafter as Tovld. In the event current through the port exceeds Icut for more than Tovld power through the port is disconnected. Current through the port is thus to be monitored and compared with Icut, however Icut is not associated with a current limiter.
A PoE controller serving a plurality of ports preferably is provided with a power management functionality which prevents powering of ports in excess of the available overall power. In an exemplary embodiment this is accomplished by providing a guard band as described in copending U.S. patent application Ser. No. 10/949,208 filed Sept. 27, 2004 entitled “METHOD AND APPARATUS FOR POWER MANAGEMENT IN A LOCAL AREA NETWORK”, the entire contents of which is incorporated herein by reference. In an exemplary embodiment the guard band is equal to, or greater than, the maximum power which may be drawn by a port, the maximum power typically being a function of the inrush current. Thus, by implementing the guard band, no port can be powered which would bring the total utilized power to be in excess of the total available power. It is to be understood that in the event the total utilized power is in excess of the total available power even during an inrush event, unless power to at least one port is disabled the power supply will overload and shut down.
The above mechanism is generally satisfactory, however in the event a small power supply is utilized, having a capacity on the order of 2-4 fully powered “af” ports, the guard band prevents utilization of a significant portion of the power supply capabilities. This is particularly problematic in the event that the attached PDs are capable of starting up with a limited inrush current, and draw power well below the maximum allowed by the “af” standard. It would be advantageous in such a circumstance to have a lower port current limit assigned, thereby limiting the inrush current to an amount less than the maximum allowed by the “af” standard. This in effect reduces the guard band and allows for powering of ports with an increased utilization of the power supply.
The “af” standard limits the amount of power available to a powered device to 12.95 watts, and devices demanding power in excess of the 12.95 watt power limit are not supported. In order to meet growing power demands, in particular demands for PDs drawing in excess of 12.95 watts, a study group entitled “IEEE 802.3at DTE Power Enhancements Task Force” has been formed, which is in the process of producing a higher power standard, hereinafter the “at” standard. While the study group has not yet finalized its recommendations, it appears that “at” standard will specify a higher current limit than the “af” standard, and the PSEs meeting the “at” standard are to support PDs meeting the “af” standard.
What is needed, and not supplied by the prior art, is a PoE controller supporting a plurality of current limits each of which may be associated with any of the ports of the PoE controller.